The present invention relates to low voltage, high current electrical power distribution systems and, more particularly, to fuse protected electrical disconnect devices for use in power distribution systems.
A common problem in various industries, for example the telecommunications industry, is the distribution of relatively low voltage, relatively high current electrical power to the various devices and equipment which require such power. Telecommunications system plants, such as telephone switching offices, are typically centralized and are, therefore, constructed on a large scale to serve many customers. Since telephone systems must be highly reliable, such plants usually have a plurality of high-capacity storage batteries to provide electrical power for operating equipment whenever power is not available from conventional sources. Often, the batteries are connected in parallel with suitable power conversion equipment which receives electrical power (typically high-voltage AC) from the commercial electrical utility and supplies converted power (low-voltage DC) for operating all equipment and for maintaining the batteries in a charge condition. Whenever power from these conventional sources becomes unavailable, power is immediately and automatically supplied from the batteries.
In such systems, several batteries and power converters are usually connected together so that large amounts of equipment are supplied with power from a large collection of power sources. It is desirable in such systems to be capable of isolating individual pieces of equipment, or small groups thereof, from the power supply to perform maintenance and installation activities. It is also desirable to provide overload protection for equipment on an individual basis. Accordingly, power is conventionally distributed to equipment from distribution panels having a plurality of individual fused disconnect devices. Each disconnect device controls power to a relatively small load--for example, a cabinet containing subscriber loop interface circuits for 100 subscribers and drawing 10-50 A in normal operation.
In the past, several fused disconnect devices have been developed to allow manual control of each load circuit and to provide overload and fault protection of each load circuit. Such devices have typically included a line side terminal for a wired connection to a power supply bus, a load side terminal for a wired connection to a load device, a housing and a removable fuse-containing holder which, when installed in the housing, provides an electrical connection between the line side terminal and the load side terminal.
Such conventional fused disconnect devices suffered from a plethora of drawbacks and limitations. For example, some of these disconnect devices suffered from an inability to test the alarm circuit while the fused disconnect device was in its operative position, i.e., when the fuse holder was inserted into the housing and the device was connected to both the load and line buses. Moreover, as more and more of these disconnect devices are used in remote sites which are not easily accessible and do not have proper lighting, it would be desirable to make the results of such an alarm test more readily ascertainable in a low-light environment.
Yet another problem with conventional fused disconnect devices is the inability to determine whether or not a fuse is actually provided in the removable fuse holder. In typical telecommunications power distribution systems, the fused disconnect devices are assembled in rows of approximately 10-20 units. However, not all of these units will necessarily be operational at any given time and units which are not operational will not have a fuse in the fuse holder. Since the removal of a fused disconnect switch from a functioning distribution unit would temporarily interrupt service, e.g., telephone service to a customer, it would be advantageous to provide a method for determining whether or not a fuse was provided in the disconnect switch without removing the fuse holder or the disconnect switch itself.
Additionally, fused disconnect devices, and other types of circuit breakers generally, have been plagued with difficulties in attachment to their intended line and load devices and the internal attachment between the housing or base unit and the removable fuse holder. Many such switch devices require, for example, adapters in order to be able to connect the switch device to the intended bus structure. Such adapters are both expensive, time consuming to use and add an additional connection to the circuit which reduces the overall system reliability. Accordingly, it would be desirable to provide a fused disconnect device which did not require any adapters and which could be readily connected to an intended bus structure with a minimum of installation effort. Further, it would be desirable to provide a removable fuse holder with an uncomplicated electrical and physical connection structure that nonetheless provided a secure connection with the base unit.
Another drawback associated with conventional telecommunication power distribution systems is the tendency to distribute power via individual bus bars and using individual load circuit protection devices. Individualizing all of these components is not cost effective and requires additional connection points which increases the number of potential points for system failure.